1. Field of the Invention
This invention relates generally to a system for controlling the frequency of electrical energization of the electrode or electrodes of apparatus for electrostatically perforating traveling webs of paper, film or a like material to provide a precisely-controlled, uniform porosity of the finished product.
2. Description of the Prior Art
The prior art has disclosed the perforation of a sheet, film or similar article by passing the sheet, typically in web form, between fixed, parallel electrodes having connected thereto a pulse generator for applying periodic pulses to a plate electrode and a ground electrode defining a gap through which the sheet passes, whereby the multiple discharges between the plate electrode and the ground electrode cause perforation of the sheet. The process variables for controlling porosity include: changes in sheet speed, pulsewidth of the energizing pulses, changes in the size of the perforation holes, the degradation of the plate and ground electrodes, change in the position of the aforementioned electrodes and therfore the spark gap established therebetween, drift in spark voltage and current, and changes in the thickness or character of the dielectric materials of the sheet passing through the electrode gap. As will be detailed below, the prior art has essentially ignored many of these parameters, whereby precise control of the sheet's porosity has been difficult, if not impossible.
In U.S. Pat. Nos. 2,528,157 and 2,528,158, there is disclosed apparatus for sensing the porosity of a web or sheet of material moving therepast and for varying the frequency or pulse output of a varactor-type pulse control circuit, whereby the frequency of the discharges between the perforating electrodes between which the sheet is drawn is correspondingly changed. However, such systems ignore the process parameter of varying sheet speed, it being understood that the sheet in the form of a web is drawn from a supply to a take-up roll, past the perforating electrodes.
U.S. Pat. Nos. 2,678,373 and 3,385,951 each disclose apparatus for perforating a moving web of material that is responsive to variations in the sheet speed to vary the rate at which the perforating discharges are generated and applied to the moving web. In U.S. Pat. No. 2,678,373, a tachometer coupled to a roller associated with the moving sheet or web of material provides an output signal indicative of the sheet speed, whereby a relay control system is variably actuated, dependent upon web speed, to selectively couple a predetermined arrangement of capacitive elements to determine the charge repetition rate of the pulse generating circuit. Such a circuit has a disadvantage in that it is not continuously responsive to variations in web speed in that for a discrete range of web speeds, the pulse control circuit is disposed in a single, non-varying configuration to set its pulse or discharge rate for that range. By contrast, U.S. Pat. No. 3,385,951 discloses a perforating apparatus including a tachometer for generating an output signal indicative of web or sheet speed, the tachometer output being applied to a voltage-controlled oscillator, the output of which is of a frequency corresponding to sheet speed. The tachometer output is adjusted by a variable potentiometer, which "provides an adjustment of the slope of the curve of power frequency versus sheet velocity". It is further disclosed that the manual adjustment of this potentiometer will "allow adjustment of the hole size or compensation for changes in web thickness". Such adjustment is carried out manually by the operator, but it is not carried out in an automatic fashion. Thus, unless the operator intervenes to adjust the process by manually varying the potentiometer, no compensation is made for changes in the control parameters such as hole size, spark pulsewidth, deterioration of the electrodes, changes in the position of the electrodes, changes in the power supply and/or changes in the characteristics of the sheet material.